Imaging is of value in distinguishing Graves disease from toxic nodular goiter, assessing the function of nodules, differentiating subacute thyroiditis and related conditions from Graves disease and toxic nodular goiter, defining occult thyroid lesions, determining the location and size of functional thyroid tissue, and detecting metastases from differentiated thyroid cancer. Perhaps the most frequent application of thyroid imaging is in evaluating the function of palpable nodules. A nodule is a palpatory or pathologic finding and cannot be defined by thyroid imaging alone. The imaging of palpable nodules allows normally functioning tissue to be distinguished from hypofunctioning (“cold”) or hyperfunctioning (“hot”) lesions. Hyperfunctional nodules (Fig. 34-1) may be associated with clinical or subclinical hyperthyroidism and are unlikely to be cancer. Hypofunctional nodules may be associated with a higher incidence of malignancy (see Chap. 39 and Chap. 40).

Imaging with or without an uptake measurement may help differentiate hyperthyroidism due to thyroiditis (viral, postpartum, or amiodarone related), from hyperthyroidism due to Graves disease or nodular goiter (see Fig. 34-1).

Occult or nonpalpable thyroid lesions may be a clinical consideration in some patients without known primary tumors and in patients exposed to therapeutic x-rays or other radiation in infancy and childhood, who are at risk of developing a thyroid neoplasm later in life.22,23

Thyroid size generally is estimated by palpation or by ultra-sonography. Size also may be assessed scintigraphically, using a rectilinear scanner that provides a full-sized 1:1 image.24 Estimates of the size of the thyroid gland or thyroid nodule are usually used to calculate the amount of therapeutic radioiodine to be administered in hyperthyroidism and to assess the response to radioiodine or suppressive therapy. The use of ultrasonography to measure thyroid volume is discussed in Chapter 35.

Imaging may help establish the presence of a nonpalpable thyroid gland, for instance in hypothyroid infants, and may help localize mediastinal, lingual, or other ectopic thyroid tissue25,26 (Fig. 34-2). Mediastinal imaging is useful to locate aberrant thyroid tissue that might manifest as a substernal mass on a chest radiograph, often with tracheal deviation.

Whole body radioiodine imaging, together with serum thyroglobulin determinations, is used to identify, locate, and monitor differentiated thyroid cancer (see Chap. 40).